1. Field of the Invention
The present invention relates to a semiconductor laser device of the resin mold package type accommodating a laser diode (hereinafter referred to as an LD) element.
2. Description of the Prior Art
Semiconductor laser devices such as LD elements are used in optical disc devices which accommodate compact discs (hereinafter referred to as CDs), or various instruments using light, such as laser beam printers. Can packages are known as examples of the semiconductor laser devices. FIG. 1 is a perspective view, partly broken away, of a can type semiconductor laser device. FIG. 2 is a partially sectional view showing an embodiment in which this can type semiconductor laser device is mounted on an instrument. The can type semiconductor laser device has a square rod-shaped heat dissipater 4 protruding upwards from the top of a disc-shaped stem 3, a submount layer 2 also serving as a radiating board, an LD element comprising an LD chip 1 provided on the submount layer 2, and a cap 6 with a glass window 5 at the top. The semiconductor laser device is of a structure in which the LD element is soldered to a side surface (main surface) of the heat dissipater 4, and the cap 6 is welded to a collar portion 3a of the stem 3 to cover and protect the parts upwardly protruding from the stem 3. On the collar portion 3a is formed a register groove 3b for use in packaging. The numeral 12' denotes a lead terminal. As shown in FIG. 2, this can type semiconductor laser device is set in place in an instrument by placing the cap 6 side of the device in a stepped accommodating hole (grooved portion) 8 formed in part of the instrument, indicated at 7, and adhesive bonding or contact bonding is used for mounting the collar portion 3a of the stem 3 at the step portion. Laser light 9 is projected in the direction of an arrow through the glass window. With this can type of semiconductor laser device, the point of emitting laser light needs to be kept at a fixed position. As shown in FIGS. 3 and 4, therefore, the LD chip 1 is adjusted to be positioned at the intersection 10 of a plane passing the centers of the circular stem 3 and the glass window 5 (both being concentric members) and perpendicular to the main surface of the heat dissipater 4 (X-axis plane) and a plane passing the centers of the circular stem 3 and the glass window 5 and perpendicular to the former plane (Y-axis plane), as viewed from above in a plan view, with the locations of the submount layer 2 and the heat dissipater 4 being determined relative to the position of the LD chip 1. When the device is to be incorporated in the instrument, the outer periphery and top surface of the collar portion 3a of the stem 3 are mounted on the groove portion 8, as shown in FIG. 2, and then adhesive bonded or contact bonded to fix the device. The shape of the semiconductor laser device with the collar portion 3a complies in practice with certain standards, since changes in the design or parts of the instrument have to be avoided. The outside diameter of the collar portion 3a of the stem 3 is 5.6 mm in the currently most mass-produced semiconductor laser devices for CDs with a low output of 3 to 5 mW, and 9 mm in other, high output, semiconductor laser devices.
Low cost semiconductor laser devices are in high demand. In recent years, resin mold type semiconductor laser devices have been developed which are lower in the cost of manufacturing and higher in the degree of freedom of shape than the can type semiconductor laser devices. FIG. 5 is a perspective view showing the shape of a resin mold type semiconductor laser device. This resin mold type semiconductor laser device has an LD chip 1 mounted on the top of a broad end portion of a lead frame 12 via a submount layer 2, and the surroundings of the front end of the lead frame, including the LD chip 1, are molded with a sealing resin member (resin mold) 11 such as a transparent epoxy resin. The sealing resin member 11 takes a cylindrical shape having a collar portion 11a corresponding to the collar portion 3a of the can type device stem 3. The numeral 13 signifies a gold wire. The resin mold type semiconductor laser device has been known as a light emitting device with a low optical density per unit area, such as an LED. Such a resin mold type semiconductor laser device having the collar portion 11a is easily mounted on an instrument by the same procedure used for the can type semiconductor laser device, and also has the advantage of low manufacturing costs because it is of a mold type.
FIG. 6 is a schematic sectional view showing the structure of the LD chip 1 used in the above mold type semiconductor laser device. The LD chip 1 has a double heterostructure (DH), which comprises an n type clad layer 15 of AlGaAs, an active layer 16 of GaAs, a p type clad layer 17, and a p type cap layer 18 laminated in this order on an n type GaAs substrate, and further has an electrode 19 on the surface side of the closed end of the p type cap layer, as well as a back electrode 20 on the rear side of the GaAs substrate 14. FIG. 7 is a sectional view taken on line A--A of the LD chip 1 shown in FIG. 6, with the same numerals being used on the portions common to both drawings. In the LD chip 1, an end face destruction preventing layer (passivation film) 22, such as silicone, with a low absorption coefficient for light in the wavelength band of laser light 9 and high thermal stability, is formed on a light emitting end face 21 which projects laser light 9. This layout makes it possible to prevent the characteristics of the sealing resin member 11, which seals the LD chip 1, from deteriorating due to optical damage. In other words, when the end face destruction preventing layer 22 resistant to laser light 9 is inserted between the light emitting end face 21 and the sealing resin member 11, the optical density of laser light 9 at the sealing resin member 11 is reduced, whereby possible damage to the sealing resin member 11, such as epoxy resin, by laser light 9 is prevented.
FIG. 8 is a plan view of the resin mold type semiconductor laser device shown in FIG. 5. FIG. 9 is a sectional view taken on line A--A of FIG. 8. The LD chip 1 in the resin mold type semiconductor laser device is positioned at the intersection 10 of the X axis and the Y axis which is at the center of the sealing resin member 11, as in the case of the can type device. Thus, the central point 23 in the thickness direction of the broad portion of the lead frame 12 bearing the LD chip 1 is away from the position of the LD chip 1 (the center of the sealing resin member 11) in the -X direction by the sum of the thickness of the submount layer 2 and a half of the thickness of the lead frame 12 (offset distance .DELTA.X.sub.off).
However, the following two major problems occur with a semiconductor laser device of the above-described structure:
(1) The position of the point of light emission varies according to temperature rises of the resin around the LD chip 1 due to a current flowing there, or changes in environmental temperature.
(2) Peeling occurs between the sealing resin member 11 and the end face destruction preventing layer 22, causing the optical radiation characteristics to deteriorate. The optical radiation characteristics are concretely expressed as a farsighted visual field pattern or far field pattern (which may be hereinafter referred to as FFP).
FIG. 10 is a graph showing the relationship between the amount of the X-direction displacement of the point of light emission and the time that elapses until the action of turning light on or off. In the drawing, a dotted line represents the data for the cylindrical resin mold type semiconductor laser device illustrated in FIGS. 8 and 9, and a solid line represents the data for a flat resin mold type semiconductor laser device to be described later. According to FIG. 10, when the cylindrical resin mold type semiconductor laser device illustrated in FIGS. 8 and 9 was activated with an operating current of 50 mA at room temperature, a marked movement in the X direction of the point of light emission was observed. As seen from FIG. 10, after the laser was turned on, the point of light emission was displaced by 0.5 .mu.m in the -X direction (in the X-axis direction, the LD chip 1 side is designated as +X, and the lead frame 12 side as -X), i.e., on the lead frame 12 side, in about 2 minutes; whereas after the laser was turned off, the point of light emission returned in about 2 minutes to the center (intersection) 10, which had been the point of light emission before turning on the light. When this semiconductor laser device was integrated into an optical pickup for CDs, the action of the CD device became disorderly immediately after the semiconductor laser device was lit up, or at the time of changes in environmental temperature. The shift in the position of the point of light emission was found to come from the positional movement in the -X direction of the lead frame 12 by thermal expansion of the sealing resin material associated with heat generation during the operation of the LD chip 1, or with changes in environmental temperature. The essential cause of this positional movement is that, as shown in FIGS. 8 and 9, the lead frame 12 having the LD chip 1 mounted thereon has an offset (.DELTA.X.sub.off) 24 from the center of the sealing resin member 11.
One of the methods for inhibiting the displacement of the point of light emission is to expose part 12a of a lead frame 12, which bears an LD chip 1, from a sealing resin member 11 to the outside, and fix the exposed portion, as shown in FIG. 11. According to this method, the exposed part 12a of the lead frame 12 is immobile even when the resin material thermally expands. Hence, the LD chip 1 borne thereon also undergoes little dislocation, thus making it possible to prevent the position of the point of light emission from moving. The aforementioned problem of resin peeling (2) is subordinate to the problem (1) and can be practically solved by molding the sealing resin member 11 into a thin flat plate, as illustrated in FIG. 11. FIG. 11 is a schematic perspective view showing the appearance of a resin mold type semiconductor laser device molded in a flat form in which part of the lead frame is exposed. With this flat type device, the volume of the resin material that covers and seals the surroundings of the end face destruction preventing layer 22 (FIG. 7) is made small, and the thickness of the resin layer of the sealing resin member around the LD element is uniformed. Thus, the amount of displacement due to thermal deformation of the resin is reduced, whereby the peeling phenomenon is suppressed. Samples were trial manufactured in which the end face destruction preventing layer 22 comprised a rubbery organosilicon resin consisting essentially of dimethylsiloxane, and the resin mold was a cylindrical resin mold as shown in FIG. 5 (Sample No. 1) or a thin flat resin mold with a partially exposed lead frame as shown in FIG. 11 (Sample No. 2). A heat cycle test of these samples was conducted, and the electrical and optical characteristics of the element were investigated in certain cycles. The conditions for the heat cycle test involved cycles each comprising heating the sample at 85.degree. C. for 30 minutes, cooling it at -40.degree. C. for 30 minutes, and then returning the sample to an endothermic reaction at 85.degree. C.
TABLE 1 ______________________________________ Sam- No. of heat ple Shape of resin Details of cycles No. mold defect 100 200 300 400 ______________________________________ 1 Cylindrical Defect in FFP 0 5 8 12 (%) Defect in other 0 0 0 0 than FFP (%) 2 Flat Defect in FFP 0 0 0 0 (Lead frame (%) exposed) Defect in other 0 0 0 0 than FFP (%) ______________________________________
As noted from Table 1, the cylindrical sample (No. 1) exhibited defects in FFP during the test, whereas the thin flat sample (No. 2) involved few defects. These defects in FFP are ascribed to the peeling of the interface between the end face destruction preventing layer 22 and the sealing resin member 11. Thus, peeling of the resin can be prevented by forming the sealing resin member 11 into a thin flat resin type.
However, the semiconductor laser device having the thin flat type sealing resin member formed with part of the lead frame exposed as shown in FIG. 11 poses the following problem:
The semiconductor laser device having a cylindrical sealing resin member is provided with a circular collar portion, and thus can be mounted on an instrument by being fitted into an accommodating hole, as can the can type device. The thin flat type sealing resin member, on the other hand, has no circular collar portion, and so cannot be mounted into an accommodating hole, unlike the conventional type.